Modern wireless communication systems use re-transmission schemes, often denoted as Automatic Repeat Request (ARQ). In an ARQ scheme a data packet, transmitted from a transmitter, is appended with a CRC. A receiver decodes the data packet, re-calculates the CRC and compares the obtained CRC with the transmitted CRC. If the CRC matches, an acknowledgement (ACK) is sent as feedback to the transmitter to indicate that the corresponding data packet was received correctly, otherwise a negative acknowledgment (NACK) is sent as feedback to the transmitter to indicate that the data packet was not received correctly. Based on such feedback (ACK or NACK) the transmitter can retransmit the corresponding data packet if the feedback was a NACK.
In case the feedback is a NACK, the time for successful data transmission is at least the time required for providing the feedback and to re-transmit the data from the transmitter. The time duration between a transmission and a consecutive re-transmission may be called a re-transmission round trip time.
In LTE and other wireless communication systems, both FEC (Forward Error Correction) encoding and ARQ may be applied, this is also known as Hybrid ARQ (HARQ). HARQ is used in HSDPA and HSUPA which provide high speed data transmission (on downlink and uplink, respectively) for mobile phone networks such as UMTS, and in the IEEE 802.16-2005 standard for mobile broadband wireless access, also known as “mobile WiMAX”. It is also used in EVDO and LTE wireless networks.
Type I Hybrid ARQ is used in ITU-T G.hn, a high-speed Local area network standard that can operate at data rates up to 1 Gbit/s over existing home wiring (power lines, phone lines and coaxial cables). G.hn uses CRC-32C for Error Detection, LDPC for Forward Error Correction and Selective Repeat for ARQ. One of the improvement areas over LTE, will in 5G communication systems be latency. In LTE, HARQ feedback is transmitted several subframes later from the receiver to the transmitter. In LTE one subframe spans 1 ms resulting in a latency of the lower layer re-transmission protocol of several ms. To reduce this duration, 5G communication systems may have a frame structure where the feedback can be sent at the end of the subframe, in which the corresponding data is transmitted. FIG. 1 depicts an example TDD frame structure enabling such feedback transmission. Since full-duplex is not yet a viable solution, the Down-Link (DL) transmission has to stop some time before the Up-Link (UL) transmission can start to enable the receivers to switch from transmit to receive and vice versa. This time, from stopping the DL transmission until UL transmission starts, may be called a guard period. The guard period may also include possible timing advance, which may be used to compensate for propagation delay and thereby provide a suitable timing for e.g. wireless devices or UEs to enable synchronization at e.g. an eNB or base station.
For FDD, system full duplex operation is feasible and consequently UL- and DL-transmissions can overlap as indicated in FIG. 2. The overlap may mean that e.g. there may be concurrent transmissions on UL and DL. As an example, retransmission feedback in a UL subframe may be transmitted while data packets are transmitted in the corresponding DL subframe. However, to be able to send the feedback signal in an UL subframe, before expiry or at the end of the DL subframe, even for FDD, the downlink transmission would have to stop early and thereby wasting DL resources at the end of the transmitted subframe. Using these “wasted” DL resources for the next DL subframe is not desirable since this creates dependency between subframes.
The frame structure outlined above and shown in FIG. 1 requires a duplex direction switch for TDD every subframe. Every duplex direction switch, with the corresponding guard period, will result in that some symbols in every DL subframe cannot be used. A similar effect is expected for FDD, as described in relation to FIG. 2, since the last symbols in a DL subframe are not used since that usage would create an inter dependency between DL subframes which is not desirable.
Therefore, for both TDD and FDD, there is a risk of not efficiently utilizing the full capacity of the channels since parts of the allocated resources cannot be used in every subframe in a communication system with a frame structure where the feedback can be sent at the end of the subframe in which a corresponding data packet is transmitted.
Providing retransmission feedback in the same subframe could—depending on the decoder implementation—be more battery consuming since the terminal has very short time to decode the received data. If the application requires short latency this is acceptable, however, if an application does not require low latency it could be beneficial to provide the terminal with more time for decoding and thus enable potential energy savings.
Consequently, if feedback is provided promptly there is a risk that the channel is not fully utilized and that the UE power consumption increases.